Researchers have linked the "hygiene hypothesis" - the idea that lack of exposure to germs, viruses and parasites harms the immune system - to rising rates of dementia in richer nations. A new study by Cambridge University compared dementia cases in 192 countries and found it was more common in those with better sanitation and less disease. Countries where everyone has access to clean drinking water, such as the UK and France, have nine per cent higher Alzheimer's rates then average.